Method for producing highly crimped regenerated cellulose fibers by solvent stretching

ABSTRACT

Highly crimped regenerated cellulose fibers can be obtained by stretching filaments containing the reaction product of cellulose xanthate and formaldehyde in a non-aqueous liquid having a boiling point of at least 80*C. and maintained at a temperature of 60*C to 150*C, relaxing said filaments in an incompletely regenerated state in an aqueous solution having a swelling action on the filaments and maintained at a temperature of 30*C to 90*C and then subjecting the filaments to regeneration treatment to complete regeneration.

United States Patent Kawai et al. 1 Sept. 5, 1972 [54] METHOD FOR PRODUCING HIGHLY 3,109,699 11/1963 Richardson .214] 198 CRIMPED REGENERATED 3,419,652 12/1968 Kubota et a1. ..264/ 168 CELLULOSE FIBERS BY SOLVENT 3,494,995 2/1970 Thomas et al. ..264/ 188 C1 2,282,568 5/1942 Finzel ..8/ 131 Inventors Amish! KIWI; M s Sumki, both 2,371,579 3/1945 C016 et a1. .....................8/132 of Ohtake, Japan 2,878,547 3/1959 McMaster et a1 ..8/1 17 3,140,957 7/1964 Tanabe et a1 ..117/6 [73] mm 2,327,516 8/1943 Fink etal ..s/130.1

Tokyo, Japan [22] Filed: June 12, 1970 Primary ExaminerJay H. W00 pp 45 722 Att0mey-Christen and Sabol [57] ABSTRACT [30] Fm Applim Priority Dam Highly crimped regenerated cellulose fibers can be ob- June 24, 1969 Japan ..44/49s34 mined y stretching filmms containing the reaction product of cellulose xanthate and formaldehyde in a 52 us. c1. .264/197, 264/168, 264/188 non-aqueous liquid having a boiling point of at least 511 1111. c1. ..no113/2a and maintained a! a mmpemm of w 581 Field ofSearch......................264/l88-198 168' relaxing said filaments inwmflelely 8/116 4 130 I 7 regenerated state in an aqueous solution having a swelling action on the filaments and maintained at a temperature of 30C to 90C and then subjecting the [56] Mamas Cited filaments to regeneration treatment to complete UNITED STATES PATENTS regeneration- 2,317,152 4/1943 Costaetal ..214/196 ISCIaIImIDrIWiBQHguR STRETCH/N6 l/V NON-AQUEOUS LIQUID CUTTING REL/1X4 T/O/V REGE/VEFPA T/ON PUFF/FICA T ION PATENTEHSEP 5 0912 3,689,622

F/LAME/VTS CO/VZ IINING HYOROXYMETHYL CELLULQSE XA/VTHATE STRETCH/N6 //v NON-AQUEOUS uou/a CUTTING RELAXA T/O/V REGENERATION PUP/FICA 7' ION This invention relates to a novel method for producing highly crimped regenerated cellulose fibers by the viscose method.

The object of this invention is to produce highly crimped regenerated cellulose fibers having water reversible micro crimps and possessing excellent fiber properties and spinning property for preparing fine count yarn. Furthermore, fabrics made from said fibers have excellent hand and bulk as compared with those from conventional ordinary rayon, polynosic fibers, high wet modulus rayon, etc. Methods for producing highly crimped regenerated cellulose fibers from which fabrics of excellent hand can be obtained were earlier proposed in U.S. Pat. Nos. 3,419,652 and 3,547,812. However, fibers obtained by these methods are difficulty spun to fine count yarn. This invention resides in overcoming said problems and can produce fibers having excellent spinning property and fiber properties. The fibers obtained according to this invention are characterized by three dimensional microcripms, more than 40 crimps per inch, and they have excellent spinning property and fiber properties.

The method of this invention comprises stretching filaments containing the reaction product of cellulose xanthate and formaldehyde in a non-aqueous liquid having a boiling point of at least 80C and maintained at a temperature of 60to l50C, relaxing said filaments in an incompletely regenerated state in an aqueous solution having a swelling action on the filaments and maintained at a temperature of 30C to 90C and then subjecting the filaments to regeneration treatment to complete the regeneration.

The characteristic of this invention resides in the combination of novel stretching conditions and relaxation conditions, but said stretching conditions are more important. That is, in the method proposed in U.S. Pat. Nos. 3,419,652 and 3,574,812, filaments containing the reaction product of cellulose xanthate and formaldehyde are stretched in an acidic aqueous solution under a tension of less than 0.3 g/d in order to form a non-symmetrical heterogeneous structure in cross section similar to conjugate fibers and then the thus stretched filaments are relaxed in a bath having a swelling action on the filaments in a state of incomplete regeneration. On the other hand, according to this invention, the filaments are stretched in a nonaqueous liquid and the stretched filaments are swollen in an incompletely regenerated state, namely at a regeneration degree of less than 89 percent, to relax them.

As mentioned above, the first point of the method of this invention is to produce the reaction product of formaldehyde and cellulose xanthate as filaments, the second point is to stretch the thus obtained filaments in a non-aqueous liquid to form a heterogeneous structure in cross section similar to conjugate fibers and the third point is to relax the stretched filaments which still contain the reaction product in a bath having a swelling action on said filaments to develop the heterogeneous structure formed in the stretching step.

The reaction between formaldehyde and cellulose xanthate takes place in accordance with the following equation l and proceeds in an acidic medium.

cellO-CS-, CH,0H cellO-CS,Ol-l 1 The amounts of xanthate and fon'naldehyde and the acid concentration of the system affect said reaction. Accordingly, the cellulose xanthate ion is unstable in an acidic state, but the formaldehyde derivative of cellulose xanthate is relatively stable in an acidic state. However, in an aqueous solution, hydrolysis as shown in the following equation (2) takes place even in an acidic state, so that the decomposition rate increases with an increase in temperature. in an aqueous solution, the decomposition rate abruptly increases when the temperature exceeds 55to C.

cell-OCS,Cl-l Ol-l l-l,0 cell-OHCS,

l-lOCl-bOl-l (2) On the other hand, in a non-aqueous medium, the formaldehyde derivative of cellulose xanthate is very stable even at high temperatures.

The heterogeneous fiber structure in cross section which is similar to that of conjugate fibers is formed in the stretching step. In order to attain effective stretching, it is necessary to increase mobility of the formaldehyde derivative molecules and to provide deviation in distribution of the molecules in the filaments. For this purpose, it is effective to change the fiber structure in a short period of time, namely, to stretch the filaments in a medium at a high temperature. However, as mentioned above, in an aqueous solution, decomposition of the formaldehyde derivative takes place at high temperatures, whereby formation of a heterogeneous structure is prevented and thus development of crimps is suppressed. According to this invention, these drawbacks are overcome since a nonaqueous liquid is used as the stretching medium. The difference between the method of this invention which uses a nonaqueous liquid as the stretching medium and the conventional method which uses an aqueous solution as the stretching medium resides in the heat transfer and material transfer between the filaments and stretching medium.

In the conventional aqueous solution, since (a) diffusion of water and acid into filaments is rapid in an acidic aqueous solution at high temperatures, (b) the formaldehyde derivative is easily decomposed with water at high temperatures, (0) the remaining cellulose xanthate ion is also easily decomposed with acid at high temperatures and (d) the fiber structure is fixed by the decomposition of the formaldehyde derivative and the remaining cellulose xanthate ion, development of the heterogeneous structure is prevented. In this invention, little material transfer to the outside of the system takes place during stretching and a high degree of stretch is possible at high temperatures.

Therefore, according to this invention, development of the heterogeneous structure is easier than according to the methods of U.S. Pat. Nos. Nos. 3,419,652 and 3,574,812. in the method of U.S. Pat. Nos. 3,419,652 and 3,574,812, in order to obtain crimps, the stretching is carried out in an aqueous solution under a stretching tension of less than 0.3 g/d, while according to this invention, stretching may be carried out under a wide tension range to develop a heterogeneous structure. Therefore, this invention provides great improvement of fiber properties, especially, tenacity and wet modulus, and spinning property as compared with the method of U.S. Pat. Nos. 3,419,652 and 3,574,812.

Furthermore, according to this invention, processability and producibility are improved.

Accompanying drawing is a block diagram illustrating the general aspects of the process of this invention.

This invention will be explained in more detail hereinbelow.

The first point of this invention is to form the reaction product of formaldehyde and cellulose xanthate (which mainly consists of hydroxymethyl cellulose xanthate) in filament form and for this purpose, various means may be employed. For example, filaments containing hydroxymethyl cellulose xanthate may be produced by extruding viscose to which formaldehyde is added or by adding formaldehyde to a coagulation bath or by treating coagulated filaments containing cellulose xanthate with an aqueous solution of formaldehyde.

The viscose used in this invention preferably contains 2 to 8 percent of total alkali. The viscose when spun must have a salt point of at least 16, preferably of 18 to 23. When formaldehyde is added to the viscose, the amount of the former is preferably 0.2 to 2 percent based on the weight of viscose.

The coagulation bath contains preferably 20 to 250 g/l sodium sulfate, less than 0.3 g/l zinc sulfate, and sulfuric acid in an amount within the range shown by the following equations:

Minimum concentration of sulfuric acid (g/l) 3/1 8 Maximum concentration of sulfuric acid (gll) 8A 16 wherein A is total alkali concentration (percent) in the viscose. When formaldehyde is not added to the viscose, the coagulation bath contains preferably 4 to 20 g/l formaldehyde. When formaldehyde is added to the viscose, the formaldehyde concentration in the coagulation bath may be 1 to 6 gll. The temperature of the coagulation bath is lower than 45C, desirably 10 to 35C.

Filaments containing hydroxymethyl cellulose xanthate may also be obtained by extruding viscose into a coagulation bath containing 14 to 50 g/l sulfuric acid, 20 to 25 gll sodium sulfate and less than 1 g/l zinc sulfate at a temperature of lower than 35C and treating thus coagulated filaments with an aqueous solution containing 15 to 70 gll formaldehyde without adding formaldehyde to the viscose or the coagulation bath.

In all the above cases, the coagulation bath or the viscose may contain various surface active agents.

The second point of this invention resides in the step of stretching the filaments containing hydroxy methyl cellulose xanthate to form the heterogeneous structure.

In this invention, a non-aqueous liquid having a boiling point of at least 80C and a weak swelling action on the filaments containing hydroxy methyl-cellulose xanthate is used as a stretching medium. When a nonaqueous liquid having a strong swelling action on the filaments are used, formation of the heterogeneous structure is markedly prevented. Typical liquids are liquid paraffins, such as normal paraffine, iso-parafflne, etc., silicone oils, such as dimethyl silicone oil, methyl phenyl silicone oil, etc., white oil (mixture of nhydrocarbon and iso-hydrocarbon of to 30 carbon atoms), glycerine, polyethylene glycol and aromatic solvents such as xylene, toluene, styrene,

dichlorobenzene, diphenyl chloride, alkyl napthalene, alkyl benzene, polyphenyl, etc., natural fat and oils, ethylene glycol, ethylene glycol ethyl ether, diethylene glycol, triethylene glycol, polyethylene glycol having a molecular weight of 200 to 800, methyl methacrylate, trioxane, etc.

A low temperature may be used if a long treating time is used. However, use of the low temperature doe not result greatly improved advantages as compared with those when stretching in an aqueous solution. Therefore, the temperature of the stretching medium of this invention is 60C to 150C, preferably C to 130C. When the stretching is carried out at a temperature higher than l00C, it is necessary to shorten the stretching time to prevent excess regeneration. Furthermore, it is apparent that the temperature is preferably below the boiling point of the liquid used. Moreover, it is also necessary to assure that the filaments while stretching do not come into contact with drops of water which are carried into the non-aqueous medium by the filaments withdrawn from the coagulation bath. Accordingly, in this invention, it is preferable to provide an apparatus to separate water from the non-aqueous liquid.

The stretch ratio is also important for formation of the heterogeneous structure.

The suitable stretch ratio for attaining the object of this invention is generally from 0.25 to 0.8 times the maximum stretch ratio at given conditions such as content of hydroxymethyl cellulose xanthate, temperature of the stretching medium, etc. The maximum stretch ratio is defined as follows: the stretch tension increases with an increase in stretch ratio, but when the stretch ratio exceeds a certain value, filaments begin to break. With an increase in number of broken filaments, the stretch tension decreases. The stretch ratio which provides said maximum stretch tension is defined as the maximum stretch ratio (percent). (Stretching to 2 times is 100 percent and to 3 times is 200 percent). When the stretch ratio is less than 0.25 times or more than 0.8 times the maximum stretch ratio, formation of heterogeneous structure is insufficient.

The filaments stretched in the non-aqueous liquid should then be subjected to a relaxation treatment in a relaxation bath while having a regeneration degree of less than 89 percent. The un-regenerated portion of the filaments after stretching mainly comprises hydroxymethyl cellulose xanthate, but when the regeneration degree is more than 90 percent and the content of hydroxymethyl cellulose xanthate is small, the effects of the relaxation treatment are not achieved and crimps are not developed. Thus, in order to obtain the effects of relaxation, the filaments should still be in a thermodynamically active state. The term regeneration degree" used herein designates a value represented by the ratio of the y-value when the regeneration degree of viscose when spun is taken as 0 percent and that of completely regenerated cellulose is percent. Thus, for example, when the y-value of viscose when spun is 80 and that of the filaments entering the relaxation bath after stretching is 32, the regeneration degree of the filaments is 60 percent in accordance with (80 32)/80 X 100 (percent). The regeneration degree of filaments entering the relaxation bath after stretching is preferably 20 to 80 percent, within which range development of crimps is particularly satisfactory.

The third point of this invention is that said stretched filaments containing hydroxymethyl cellulose xanthate are relaxed to develop the heterogeneous structure formed during stretching. The relaxation medium and its temperature are important as relaxation conditions. The following three kinds of relaxation baths are preferably used as a medium having a selling action on the filaments.

1. An aqueous solution at 30 to 90C As said aqueous solution, water or water containing a small amount of an acid may be used and it may further contain surface active agents or other agents. The pH of the aqueous solution is preferably 2.0 to 10.5. If the pH is less than 2.0, swelling of hydroxymethyl cellulose xanthate and the development of crimps are not sufficient. On the other hand, if the pH is more than l0.5, the filaments are swelled to a great extent and begin to be dissolved. The pH is more preferably 3.0 to 8.0. The temperature of the aqueous solution is particularly preferably 40C to 80C.

2. An aqueous solution containing inorganic salts or organic salts or mixtures thereof As said salts, alkali metal salts, alkaline earth metal salts or ammonium salts of inorganic or organic acids or mixtures thereof may be employed. Typical salts are sodium acetate, potassium tartarate, sodium sulfate, potassium thiocyanate, potassium hydrogen phosphate, magnesium chloride, sodium chloride, etc. In such aqueous salt solutions, swelling of the filaments containing hydroxymethyl cellulose xanthate is much greater and the effect of relaxation is greater than in an aqueous solution containing no salts. In such aqueous salt solutions, the effect of relaxation can be attained even at about a pH of 1.0, but the preferable pH value of the solution is 1.5 to 8.0. The concentration of the salts is preferably 1 to 100 g/l, particularly preferably 3 to 50 g/l. Salts of heavy metals such as zinc, cadmium, copper, nickel, cobalt, etc. tend to restrain swelling of the filaments. However, presence of small amount of said heavy metal salts results in prevention of excess swelling of the filaments and is effective for obtaining fibers of balanced properties. The concentration of said metal salts is less than I g/l, desirably less than 0.5 g/l.

The temperature of the solution is desirably 30 to 90C, especially 40 to 80C as with solution 1).

In the above solutions (1) and (2), formaldehyde may be present due to partial decomposition of hydroxymethyl cellulose xanthate. However, since formaldehyde tends to prevent swelling of the filaments, it is necessary to accelerate relaxation of the filaments by increasing the temperature of the relaxation bath as the concentration of formaldehyde increases in the bath.

3. An organic solvent Typical organic solvents are nitrogen containing solvents such as formamide, dimethylformamide, dimethylacetamide, pyridine, acetonitrile, glu- Filaments subjected to relaxation treatment as described above should then be subjected to a regeneration treatment because a considerable amount of hydroxymethyl cellulose xanthate still remains, although a portion of the hydroxymethyl cellulose xanthate is decomposed in the relaxation bath. The regeneration treatment may be carried out in an acidic aqueous solution at a high temperature or in a steam at a high temperature. The completely regenerated filaments are then scoured and dried by conventional methods. In this invention, crimps are developed during relaxation and further crimps, especially micro crimps, are additionally developed during drying. The crimps can be made latent by de-crimping such as by imparting some tension to the filaments during the drying step or by stretching the crimped filaments after they are dried.

As explained above, the fibers obtained by the method of this invention have water reversible micro crimps, have excellent mechanical properties and further have a wet modulus which is markedly higher than ordinary crimped rayon. Therefore, fabrics made from fibers according to this invention have excellent strength and bulk, comfortable hand and high dimensional stability. Furthermore, since they have extremely excellent spinnability, spinning of fine count yarn is easy and they are suitable for blend spinning with synthetic fibers. Moreover, durable flame resistance can easily be imparted by blending, e.g., water insoluble organic phosphorus compounds into the viscose.

Since the fibers of this invention have properties similarv EXAMPLE 1 A viscose containing 7 percent cellulose and 3.5 percent alkali and having a viscosity of 180 poises, a salt point of 21.0 and a y-value of 76 was extruded into a coagulation bath containing 20 g/l sulfuric acid, 50 g/l sodium sulfate, 16 g/l formaldehyde and 0.] g/l zinc sulfate at 25C. The obtained filaments were stretched to 290 percent (maximum stretching ratio was 400) the original length in a bath comprising triethylene glycol at l 10C. The travel time through the bath was about 2 seconds.

The stretched filaments were sufficiently relaxed in water at C in the form of tow to develop crimps. Thereafter, they were regenerated in an aqueous bath containing 1 g/l sulfuric acid at 90C. The regeneration degree before relaxation was 60 percent.

The properties of the obtained fibers (A) are shown in Table 1. For comparison, fibers (B) were produced in the same manner, except that the coagulated filaments were stretched to I percent the original length in an aqueous solution containing 1 g/l sulfuric acid at 60C, regeneration degree before relaxation was 40 (gld) g/d) (gld) tion (1:) tiont (B/d) inch) A 31 3.42 3.30l2.5 14.0 1.75 Si B 32 2.6 2.1 13.5 18.0 1.20 58 The number of crimps was obtained from the average number of crimps calculated on 50 fibers with a microscope of 8 magnification.

EXAMPLE 2 A viscose containing 8 percent cellulose and 4.5 percent alkali and having a viscosity of 120 poises, a salt point of 220 and a 'y-value of 82 was prepared. A 37 percent aqueous solution of formaldehyde was added to said viscose so that the viscose contained 1.5 percent of formaldehyde based on the weight of the viscose. This viscose was extruded into a coagulation bath containing 50 g/l sodium sulfate, 37 g/l sulfuric acid and 20 g/l formaldehyde at 40C.

The filaments withdrawn from the coagulation bath were stretched to 300 percent the original length while in a hot bath comprising white oil fraction of high boiling point from petroleum) heated to 95C. The maximum stretching percentage in this case was 360 percent.

The stretched filaments were wound on a roller and cut to staples lengths. Then, they were relaxed in an aqueous solution containing percent acetone before relaxation and kept at 40C to develop crimps. The regeneration degree was 45 percent. Thereafter, the filaments were subjected to regeneration under the same conditions as in Example 1. Fiber properties of the obtained fibers are shown in Table 2.

A viscose containing 7.5 percent cellulose and 4 percent alkali and having a viscosity of 220 poises and a salt point of 21.5 was extruded into a coagulation bath containing 30 g/l sulfuric acid, 80 g/l sodium sulfate, 0.1 g/l zinc sulfate and 10 g/l formaldehyde and kept at 30C. Thus obtained filaments were stretched to 200 percent the original length in liquid paraffin at 90C. The stretched filaments were cut to staple lengths and then relaxed in water at 60C. The regeneration degree before relaxation was 70 percent. Then, they were subjected to a regeneration treatment in an aqueous bath containing 2 g/l sulfuric acid at 85C. The obtained fibers are fibers (A) in Table 3.

For comparison, fibers (B) were produced in the same manner as above, except that the filaments were stretched to 150 percent (maximum stretching ratio was 350) the original length in an aqueous solution containing 2 g/l sulfuric acid and kept at 60C and the regeneration degree before relaxation was 40 percent. Fibers( A) and (B) were spun by a cotton system to obtain yarns having a cotton count of 45 cc. The number of yarns breaking during the spinning of fibers A was 10/420 spindles/hm, while that of fibers (B) was 19/420 spindles/hr. Table 3 shows fiber properties of single fibers and those of yarns of fibers A). and (B).

The yarns( A) and (B) were knitted to obtain fabrics, respectively. Both fabrics made from yarns (A) and B) had high bulk and firm hand. On the other hand, knitted fabrics made from ordinary crimped rayon and commercially available polynosic fibers under the same conditions as above were much inferior in hand.

EXAMPLE 4 A viscose containing 7 percent cellulose and 4 percent alkali and having a viscosity of 260 poises and a salt point of 22.5 was extruded into a coagulation bath containing 30 g/l sulfuric acid, 60 g/l sodium sulfate and 0.05 g/l zinc sulfate and kept at 20C. Thus obtained filaments were immediately passed through an aqueous solution containing 45 g/l formaldehyde, 13 g/l sulfuric acid and 30 g/] sodium sulfate at 25C and then stretched to I percent (maximum stretching ratio was 270 percent) in polyethylene glycol (molecular weight of 600) at C. The stretched filaments had a regeneration degree of 60 percent and were relaxed in an aqueous bath containing 0.1 g/l sulfuric acid, 10 g/l sodium sulfate, 0.05 g/l zinc sulfate and 3 g/l formaldehyde and kept at 55C. Regeneration was carried out in an aqueous bath containing 3 g/l sulfuric acid at 85C. Fiber properties of the obtained fibers are shown in Table 4.

Dye Number of exhaustion crimps (per inch) 63 56 What is claimed is:

l. A method for producing highly crimped regenerated cellulose fibers, which comprises stretching filaments containing the reaction product of cellulose xanthate and formaldehyde in a non-aqueous liquid selected from the group consisting of liquid paraffin, silicone oil, white oil, glycerine, polyethylene glycol, xylene, toluene, styrene, dichlorobenzene, diphenyl chloride, alkyl naphthalene, alkyl benzene, polyphenyl, natural fats and oils, ethylene glycol ethyl ether, methyl methacrylate and trioxane, said liquid having a boiling point of at least 80C and maintained at a temperature of 60 to 150C, relaxing said filaments in an incompletely regenerated state in an aqueous solution having a swelling action on the filaments and maintained at a temperature of 30 to 90C and then subjecting the filaments to regeneration treatment to complete regeneration.

2. A method according to claim 1, wherein the stretching is carried out at a temperature of 70 to 130C.

3. A method according to claim 1, wherein relaxation is carried out with the filaments having a regeneration degree of less than 89 percent.

4. A method according to claim 3, wherein relaxation is carried out with the filaments having a regeneration degree of to 80 percent.

5. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution maintained at a pH of2.0 to 10.5.

6. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution containing a member selected from inorganic and organic salts.

7. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution containing an organic solvent.

8. A method according to claim 6, wherein said salt is at least one compound selected from the group consisting of alkali metal salts, alkaline earth metal salts and ammonium salts of inorganic and organic acids.

9. A method according to claim 7, wherein said solvent is at least one member selected from the group consisting of nitrogen containing solvents selected from formamicle, dimethylformamide, dimethylacetamide and pyridine, cyclic ether compounds selected from tetrahydrofuran and dioxane, sulfur containing solvents selected from dimethylsulfoxide and dimethyl sulfone and water soluble ketones.

10. A method according to claim 1, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and having a salt point of at least 16 into a coagulation bath containing 4 to 20 g/l formaldehyde, 20 to 250 g/l sodium sulfate, less than 0.3 g/l zinc sulfate and sulfuric acid in a concentration defined by the following equations:

Minimum concentration of sulfuric acid (g/l) 3A 8 Maximum concentration of sulfuric acid (g/l) 8A 16 wherein A is alkali concentration (percent) in the f A method according to claim 1, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and 0.2 to 2 percent formaldehyde based on the weight of the viscose and having a salt point of at least 16 into a coagulation bath containing 1 to 6 g/l formaldehyde, 20 to 250 g/] sodium sulfate, less than 0.3 g/l zinc sulfate and sulfuric acid in a concentration defined by the following equations:

Minimum concentration of sulfuric acid (g/l) =3A 8 Maximum concentration of sulfuric acid (g/l) 8A 16 wherein A is alkali concentration (percent) in the viscose.

12. A method according to claim I, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and having a salt point of at least 16 into a coagulation bath containing l4 to 50 g/l sulfuric acid, 20 to 50 g/] sodium sulfate, less than 1 g/l zinc sulfate at a temperature lower than 35C and treating the resultant filaments with an aqueous solution containing l5 to g/l formaldehyde.

13. A method according to claim 1, wherein said stretching is carried out at a stretch ratio of 0.25 to 0.8 times the maximum stretch ratio. 

2. A method according to claim 1, wherein the stretching is carried out at a temperature of 70* to 130*C.
 3. A method according to claim 1, wherein relaxation is carried out with the filaments having a regeneration degree of less than 89 percent.
 4. A method according to claim 3, wherein relaxation is carried out with the filaments having a regeneration degree of 20 to 80 percent.
 5. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution maintained at a pH of 2.0 to 10.5.
 6. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution containing a member selected from inorganic and organic salts.
 7. A method according to claim 1, wherein said relaxation is carried out in an aqueous solution containing an organic solvent.
 8. A method according to claim 6, wherein said salt is at least one compound selected from the group consisting of alkali metal salts, alkaline earth metal salts and ammonium salts of inorganic and organic acids.
 9. A method according to claim 7, wherein said solvent is at least one member selected from the group consisting of nitrogen containing solvents selected from formamide, dimethylformamide, dimethylacetamide and pyridine, cyclic ether compounds selected from tetrahydrofuran and dioxane, sulfur containing solvents selected from dimethylsulfoxide and dimethyl sulfone and water soluble ketones.
 10. A method according to claim 1, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and having a salt point of at least 16 into a coagulation bath containing 4 to 20 g/l formaldehyde, 20 to 250 g/l sodium sulfate, less than 0.3 g/l zinc sulfate and sulfuric acid in a concentration defined by the following equations: Minimum concentration of sulfuric acid (g/l) 3A + 8 Maximum concentration of sulfuric acid (g/l) 8A + 16 wherein A is alkali concentration (percent) in the viscose.
 11. A method according to claim 1, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and 0.2 to 2 percent formaldehyde based on the weight of the viscose and having a salt point of at least 16 into a coagulation bath containing 1 to 6 g/l formaldehyde, 20 to 250 g/l sodium sulfate, less than 0.3 g/l zinc sulfate and sulfuric acid in a concentration defined by the following equations: Minimum concentration of sulfuric acid (g/l) 3A + 8 Maximum concentration of sulfuric acid (g/l) 8A + 16 wherein A is alkali concentration (percent) in the viscose.
 12. A method according to claim 1, wherein said filaments are obtained by extruding a viscose containing 2 to 8 percent total alkali and having a salt point of at least 16 into a coagulation bath containing 14 to 50 g/l sulfuric acid, 20 to 50 g/l sodium sulfate, less than 1 g/l zinc sulfate at a temperature lower than 35*C and treating the resultant filaments with an aqueous solution containing 15 to 70 g/l formaldehyde.
 13. A method according to claim 1, wherein said stretching is carried out at a stretch ratio of 0.25 to 0.8 times the maximum stretch ratio. 